Carbon black is widely used as a reinforcing agent in rubber. Conventionally, carbon black or like reinforcing pigments, such as silica, have been incorporated into rubbery polymers by vigorous high shear milling in a suitable mixer. However, during commercial mixing operations of this type the carbon black tends to agglomerate in the rubber thereby producing large clumps of pigment and an unsatisfactory degree of pigment dispersion.
Recently processes of the type wherein the carbon black is dispersed in the latex form of the rubber have found wide use in the rubber industry and have overcome many of the disadvantages of conventional dry mixing.
U.S. Pat. No. 3,317,458 discloses a process for the production of a vulcanizable elastomer wherein carbon black and a light reinforcing filler, i.e. silicon dioxide, are added to the latex of the elastomer. The carbon black and light filler components are mixed with said latex in the form of an aqueous suspension.
U.S. Pat. No. 3,298,984 is concerned with a method of reinforcing a rubber in its latex form which comprises preblending the reinforcing agent with the carrier and then blending the preblend of carrier and reinforcing agent with the rubber latex to thereby produce a reinforced rubber latex. The carrier or dispersing agent is a water soluble phenolic resin, particularly the condensates resulting from the reaction of an aldehyde with a phenol are desirable carriers or dispersing agents in bringing about the intimate association of the reinforcing agent with the rubber particles of the latex. Specific carriers or dispersing agents such as the condensates resulting from the reaction of formaldehyde and a polyhydric phenol such as resorcinol are disclosed.
United Kingdom Pat. No. 1,125,801 relates to the production of a carbon black masterbatch of natural rubber formed by the mixing of carbon black with rubber in latex form followed by the simultaneous coagulation together of rubber and carbon black. The carbon black is stirred into water containing a dispersing agent. Dispersing agents, such as the sodium salt of methylene dinaphthalene sulfonic acid sold under the trademark of "Dispersol LN" and a sodium salt of the sulphonation product of lignin which is sold under the Trade Name of Marasperse CB are disclosed.
U.S. Pat. No. 3,021,226 discloses a method for preparing carbon black slurries using a rosin acid soap as the dispersing agent which comprises mixing the carbon black with water in the presence of at least 5 parts by weight of a rosin acid soap and at least 0.2 parts by weight of an alkali metal or ammonium hydroxide per 100 parts of carbon black.
U.S. Pat. No. 3,023,188 discloses carbon black slurries prepared by dispersing carbon black in alcohol and a minor amount of an alkali metal hydroxide.
U.S. Pat. No. 2,794,749 discloses a method for dispersing furnace carbon blacks in aqueous medium with the aid of a dispersing composition comprising a tannic acid product, a lignin material selected from the group consisting of lignin and sulfonated derivatives thereof and an alkaline material selected from the group consisting of alkali metal and ammonium hydroxides.
U.S. Pat. No. 4,098,715 discloses a process for blending liquid rubber, i.e. cements with carbon black. The invention describes a process for blending liquid rubber in carbon black with a high torque mixer. The liquid rubber (rubber dissolved by an organic solvent) is mixed with a relatively large amount of carbon black to prepare a preliminary dispersion having a relatively high uniformity which is then diluted with liquid rubber to give a composition having a desired carbon black content. This is conducted using mixers normally used for viscous liquids such as Banbury mixers or mills.
None of the references describe or suggest the use of a low molecular weight (500-30,000) butadiene/methacrylic acid polymer as a dispersing agent for the incorporation of carbon black in a rubber latex. The process of the present invention provides for blending of latex rubber or rubber in the latex form with carbon black so as to prepare a composition in which the carbon black is uniformly dispersed in the latex without the use of high torque mixers. Further, the art does not suggest the superior dispersions that can be obtained through the process of the present invention. High shear mixing of the rubber is not required to obtain pigment dispersions and the present process can be used for carbon black, silica pigments and other fillers or pigments. In addition, the prior art methods which utilize dispersing agents generally result in a loss in physical properties of the final rubber product. The present invention allows for higher solids black dispersions of excellent stability to be prepared which do not require high shear mixing and do not diminish the final rubber properties.
It is the applicant's unsupported belief that the process of the present invention provides excellent final vulcanizate properties due to the ability of the butadiene portion of the dispersing polymer to enter into the curing reaction.